A power factor correction circuit is a form of AC to DC converter. FIG. 1 illustrates an example of an AC to DC converter 2 implemented by a power factor correction system 20 that includes a power factor correction (PFC) circuit 21 and a control unit 23. The power factor correction (PFC) circuit 21 is connected to one or more AC sources 10 to generate one or more DC outputs, here Vout+ and Vout−. PFC circuits are often used for high voltage applications, such as a power distribution network or a charging system for an electrical vehicle. The power factor of an AC electrical power system is defined as the ratio of the real power (the capacity of the circuit for performing work) supplied to a load to the apparent power (the product of the current and voltage of the circuit) in the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power. Power factor correction (PFC) is the use of power electronics to change the waveform of current drawn by a load to improve the power factor of such AC to DC converters and are used across a wide range of applications.
The control unit 23 can be a closed-loop control unit that accepts at its inputs the received AC current iac, the received AC voltage Vac, and DC bus voltage levels (Vout+ and Vout−) that are the system's output. The control unit 23 generates a set of switching control signals used by the power factor correction circuit 21 to improve the power factor by aligning phases of the received current iac and received voltage Vac.
FIG. 2 shows an example of a PFC circuit 121, which can be used as the PFC circuit 21 in FIG. 1. In the PFC circuit 121 of FIG. 2, an AC source 101 is connected through input inductor 103 to an input node N. The input node N is connected to the top and bottom lines supplying Vout+ and Vout− (which can also be referred to as first and second output nodes) through respective diodes 111 and 113. The DC output of the PFC circuit 121 is taken between these nodes. The input node N is also connected through a switch Q 115 to an intermediate node M of a voltage divider formed of the capacitors 127 and 129 connected in series between the Vout+ node and the Vout− node. (In this discussion, Vout + and Vout− are used to refer both to these nodes and their respect voltages levels.) In a typical arrangement, the capacitors 127 and 129 will have the same capacitance so that the voltage level on the intermediate node M will be midway between the Vout+ and Vout− levels. During the positive half cycle of Vac and when the switch Q 115 is off, the voltage on the input node N is passed by diode 111 to the Vout+ node. During the negative half cycle of Vac and when the switch Q 115 is off, the voltage on input node N is passed by diode 113 to the Vout− node. In either half cycle, when switch Q 115 is on, the input node N is set to the intermediate voltage level (e.g., ground, or some voltage offset from ground). The control circuitry (23, FIG. 1) of the system generates the control signal Vcontrol for the switch Q 115 to modulate the waveform on the input node N to perform the power factor correction.
As PFC circuits are often used with high voltage levels, the diodes 111 and 113 need to be able to support the high voltage levels involved. For example, a typical implementation of the PFC circuit 121 for FIG. 2 may require the diodes to take a full bus voltage of 1200V. Although 1200V SiC diodes are available, they tend to have high cost, high conduction losses, and low efficiency. Consequently, it would be useful to have a PFC circuit capable of handling such high voltage levels, but without the high cost, high conduction losses, and low efficiency associated with use of such diode elements.